The present invention relates to communication network service using a signalling system capable of transmitting a call control signal together with additional information, independently from a call. More particularly, the present invention relates to a method of displaying called party information on a calling party terminal and a communication network using the method, suitable for efficient use of circuits.
There is known a common channel signalling system which is one of the communication systems capable of processing a number of call control signals. A common channel signalling system presently used is Signalling System No. 7 recommended by International Consultative Committee in International Telegraphy and Telephony (CCITT). Signalling System No. 7 itself does not have many specifications regarding additional services. As services for an unsuccessful call, there is a service called CCBS (Completion of Calls to Busy Subscriber). As described in CCITT, Red Book, Vol. E VI, Fascicle VI.8 (1984), pp. 80-82, p. 240, CCBS is a so-called call-back service. For a user of this service function, if a telephone call is made to a busy subscriber line or a busy extension line, the system calls both the calling and called party after the busy state of the subscriber line or extension line is released. Such a CCBS function is effective only for a busy subscriber line or a busy extension line. In addition to such a busy state, even in the free state of a subscriber line or extension lines, there are, however, other cases where a call is repetitively made in vain, for example if a called party leaves his seat for conference or is out of office for several days.
Further, it can be considered the case where a calling party wishes to confirm the information on a called party such as his name before the calling party makes an actual call. Such service is not provided in Signalling System No. 7.
Similar problems are associated with Integrated Services Digital Network (ISDN), although this network can process a number of call control signals.
Japanese Patent Laid-open Publication JP-A-62-263743 discloses the technology wherein means is provided for storing a transfer telephone number for dealing with an absence of a user of an extension telephone so that call reception is switched to the transfer destination. This technology however does not solve the above-described problems.
A technology associated with a private branch exchange (PBX) is disclosed in U.S. patent application Ser. No. 07/381,190 filed on Jul. 18, 1989, now U.S. Pat. No. 4,969,182 by the same assignee, claiming the priority right based on Japanese Patent Application Serial No. 63-178233 filed on Jul. 19, 1988. According to this technology, there is disclosed a call set-up system wherein the information on a subscriber such as out-of office, attending conference and the like, is stored in PBX, and upon reception of a call, a sound (talkie) audible for a user of a calling party extension line is generated without connecting the called party extension line. This system, however, is not concerned with communications among a plurality of exchanges.